Police believe two men suspected of murdering at least four women were probably wearing the GPS tracking devices that a court mandated because they had previously sexually assaulted children, the Los Angeles Times reports.

A slew of murders, including a possible fifth victim, have been tied to Franc Cano, 27, and Steven Dean Gordon, 45, both of whom did time for sexually assaulting kids under 14.

The men were arrested Friday near a trash-sorting facility, where the body of one of the victims had previously been found on a conveyor belt, according to the Times. The four women who've been identified all had ties to prostitution and vanished in Santa Ana and Anaheim.

The arrest of the two convicted sex offenders is particularly disturbing because of the police's belief that they were wearing the GPS tracking devices while committing heinous crimes. Obviously, the main point of GPS trackers is to ensure offenders don't re-offend.

It's not clear what kind of devices Cano and Gordon may have been wearing, or if they may have malfunctioned. If Gordon and Cano weren't on some kind of house arrest, they may not have set off any red flags by allegedly committing crimes in the vicinity of their homes.

Still, California has had problems with GPS tracking of inmates in the past. The Times has previously reported that the state's corrections officials had tested some of the devices and found their locations were sometimes off by several miles.

Sex offenders in California are required to wear GPS devices for life under a sweeping 2006 law that sought to crack down on child predators.